Inorganic pigment particles (in particular titanium dioxide particles) have been widely used in various applications including coating, ink, paint, stain, and so on. For example, titanium dioxide particles are added as the pigment to coating compositions, serving to provide the coatings formed therefrom with opacity to enhance hiding power thereof
Titanium dioxide particles that are used as pigment typically have a relatively small particle size, e.g., in order of micrometers or even nanometers. Using such small particles often presents a problem that the particles are prone to aggregate during their mixing and dispersing. Aggregation of particles degrades the scattering performance thereof and hence the opacity of the coating. To reduce the aggregation of inorganic pigment particles, U.S. Pat. No. 5,385,960 discloses a process for spacing inorganic pigment particles (e.g., titanium dioxide particles) by using polymeric latex particles. It is described in the process that a dispersion of titanium dioxide in an aqueous medium is mixed with a dispersion of polymeric particles, whereby the polymeric particles are adsorbed onto the titanium dioxide particles to serve to space the titanium dioxide particles from each other. However, the thus-formed composite and as well coating compositions comprising such composite have a problem that the viscosity thereof is increasing over time. To solve this problem, the solution has been proposed that an effective amount of small-molecule dihydrogen phosphate salt is incorporated therein to suppress the constant increase of the viscosity. However, such additional incorporation of a small molecule substance presents many problems, such as environmental problem, bleeding out of small molecules, and so on.
Additionally, titanium dioxide particles used as pigment, especially titanium dioxide particles having the optimum particle size and particle size distribution for light scattering are usually one of the most expensive ingredients in the formulation of coating compositions. Therefore, there is a constant need for minimizing the amount of titanium dioxide particles used in coating compositions while the resultant coatings can still obtain the desirable opacity. To this end, the dispersibility of titanium dioxide particles in coatings needs to be improved to reduce aggregation of the titanium dioxide particles.
Therefore, the coating industry still has a need for an improved aqueous latex useful for spacing inorganic pigment particles (especially titanium dioxide particles).